It is often desirable to monitor the level of liquid in a tank or other container. For example, such apparatus is used to monitor the level of liquid in a gasoline storage tank so that, when gasoline is drawn from the tank into a tank truck for example, the amount of the gas drawn off and thus its value can be determined with accuracy. Obviously, the more expensive the liquid being sensed, the more important it is to measure accurately the changing liquid level.
There are various known apparatus for accomplishing such liquid level measurement. The type with which we are primarily concerned here employs a sensing plate which floats on the surface of the liquid. The plate is suspended on a cable from a rotary drum connected to the shaft of a servomotor. Either the motor shaft is connected to the drum by way of a flexible connection or the motor itself is mounted to the apparatus housing by way of a rotative flexure. The apparatus is arranged so that the upward force of the liquid on the sensing plate produces a balance of moments at the drum.
When the upward force on the sensing plate increases or decreases due to a raising or lowering of the liquid level, the balance of moments is upset causing angular deflection of the drum and the motor shaft or motor itself relative to the apparatus housing. The amount and direction of such deflection is detected by a sensing device which generates a signal whose magnitude and polarity reflect such deflection. That signal is used in a closed servoloop to drive the servomotor in one direction or the other to raise or lower the sensing plate as required to restore the balance of moments on the drum. Indicating devices driven by the servomotor provide a visual indication of the position of the sensing plate and thus the level of the contained liquid at any given time.
It is also known from DE-OS No. 28 53 360 that in such liquid level measuring apparatus, the drum may be flexibly coupled through a spring to a coaxially supported pivot mounting in the apparatus housing, with the pivot mounting, in turn, being frictionally connected through a gear unit to the servomotor drive shaft which also drives the indicating device. The torsional moment exerted on the drum when the sensing plate rests on the liquid is arranged to be just balanced by the spring tension. When the level of the liquid changes, the moment contributed by the sensing plate is increased or decreased and the spring rotates the drum relative to the pivot mount accordingly. That change in torsional moment is ascertained by means of a contactless signalling device which activates the servomotor by means of a control unit at a speed and direction that rotates the drum until the balance of moments is restored.
As contactless signaling devices in such apparatus, there are associated with the two relatively rotating members two slotted or apertured plates which generate in conjunction with an electrostatic or magnetic field or with light sources and suitable light sensors, voltage signals corresponding to the angle of rotation of the drum. These voltage signals are fed through a motor speed control unit as well as through a comparator stage that ascertains the direction of rotation, to a control device that activates the servomotor drive.
There is further known liquid level measuring apparatus having an explosion-proof construction in which the drive unit arranged in an encapsulated housing chamber is connected through a magnetic coupling with the cable drum. In this type apparatus, the servomotor is mounted to a pivot mount that is coaxially supported to the motor shaft. The pivot mount is flexibly connected in both directions of rotation to the instrument housing. Also, the motor shaft is connected to a roller counter via a gear drive that drives the counter with a suitable reduction ratio. A potentiometer is positioned at the pivot mount and engaged by a pin attached to the housing in order to generate a control signal proportional to the deflection of the pivot mount relative to the housing, which control signal is used to control the servomotor.
The measuring accuracy of such prior apparatus is, however, impaired due to various influences. Waves are formed at the surface of the liquid as the liquid flows into the tank or because of agitators or the like resulting in inaccurate and mostly unreproducible liquid level measurements. Besides the disturbances caused by wave motion, there also may occur oscillations or vibrations of the mechanical parts of the apparatus such as the servomotor, rotative flexures, gears, etc. These may give rise to moment-upsetting forces and introduce disturbance variables into the control loop and thus upset or impair the accuracy of the control signal for the servomotor. Other measuring inaccuracies are caused by the fact that the torsional moment on the drum contributed by the sensing plate changes as the liquid level changes because of the varying weight of the unrolled cable segment supporting the plate. In addition, the absolute encoders used in such prior apparatus to generate a remote indication of the liquid level are quite expensive, while contacting-type absolute encoders have poor wear resistance and are prone to failure. Furthermore, it is difficult to provide a calibration or zero setting for the remote display encoders in such prior apparatus at start-up or after a power failure. In general, the prior liquid level measuring instruments of this general type are disadvantaged in that they do not indicate rapid changes in liquid level quickly enough and yet with the requisite accuracy to maintain close control over the liquid inventory.